1. Field of the Invention
The present invention relates to a light source, and more particularly to a light source having a low ripple and high gain.
2. Description of the Related Art
A wavelength-division multiplexed passive optical network (hereinafter, referred to as “WDM-PON”) is a communication system for connecting a plurality of subscribers with a central office through an optical fiber. This system is also advantageous as it can easily expand communication lines. The WDM-PON includes a central office for generating downstream optical signals, a plurality of subscribers for receiving the downstream optical signals and a remote node linked to the central office through a single optical fiber and connected to each of the subscribers. The system also allows the users to transmit respective upstream optical signals to the central office.
The remote node demultiplexes the downstream optical signals received from the central office according to the respective wavelengths and outputs the demultiplexed optical signals to corresponding subscribers. Also, the remote node multiplexes the upstream optical signals inputted from the respective subscribers and outputs the multiplexed optical signals to the central office.
In the above-mentioned WDM-PON, it is necessary that the central office generate a plurality of downstream optical signals having different wavelengths in order to provide the respective downstream optical signals to each of the subscribers.
An optical transmitter for generating the downstream optical signals may include a plurality of laser light sources capable of generating optical signals having different wavelengths. The optical transmitter may also include either a spectrum-sliced light source or a wavelength-locked light source. The spectrum-sliced light source divides light of a broadband wavelength into several channels having different wavelengths, and modulates each of the channels into a downstream optical signal. The wavelength-locked light source generates downstream wavelength-locked optical signals by using each of the channels having different wavelengths.
The spectrum-sliced light source and the wavelength-locked light sources may include a light emitting diode (LED), a superluminescent LED, or an erbium-doped fiber amplifier, or other similar device(s) in order to generate light of a broadband wavelength. The erbium-doped fiber amplifier can stably generate light that has no connection with polarized light. But the erbium-doped fiber amplifier has problems in that it generates only a limited wavelength band and has a large size and a high manufacturing cost.
The LED and the superluminescent LED can generate light of a wide wavelength range but problems are known to incur as they have a low output power and they are influenced by polarized light.
FIG. 1 is a view illustrating a construction of a conventional broadband light source including a reflective semiconductor optical amplifier. As illustrated the conventional broadband light source includes a semiconductor optical amplifier 110, an anti-reflection layer 130 on a first end of the semiconductor optical amplifier 110, and a high-reflection layer 120 on a second end of the semiconductor optical amplifier 110. The semiconductor optical amplifier 110 includes a gain medium 111 for generating spontaneous emission light and clads 112a and 112b formed around the gain medium 111. The spontaneous emission light generated by gain medium 111 is reflected from the high-reflection layer 120 and is outputted to the outside through the anti-reflection layer 130. Hence, the semiconductor optical amplifier 110 amplifies the light reflected from the high-reflection layer and outputs the amplified light to the anti-reflection layer, thereby having an advantage of improving amplification efficiency.
However, because the reflective semiconductor optical amplifier includes the anti-reflection layer 130 coated on the first end and the high-reflection layer 120 coated on the second end, a so-called resonance phenomenon may occur while the light reflected from the high-reflection layer 120 is progressing through the gain medium 111. That is, there is a problem in that as reflectance of the high-reflection increases, the intensity of light amplified in the reflective semiconductor optical amplifier increases, and gain ripple of amplified light increases simultaneously. In a broadband light source for generating light of a broadband wavelength having predetermined flatness, the gain ripple is a primary factor that disturbs the flatness of light outputted from the broadband light source.